1. Field of the Invention
The present invention relates to a fuser suitable for thermal fusing for use in electrophotography, electrostatic recording methods, and magnetic recording methods, and the present invention also relates to an image forming apparatus using a toner for electrostatic image development.
2. Description of the Related Art
Hitherto, various methods of electrophotography are known, as disclosed, for example, in U.S. Pat. No. 2,297,691, Japanese Patent Publication No. 42-23910, and Japanese Patent Publication No. 43-24748. Generally speaking, an electrostatic latent image is formed on a photosensitive member by various means by utilizing a photoelectric material. Then, the latent image is developed by using toner, and the toner image is transferred to a transfer material such as paper by using a direct or indirect means as necessary. After this, the image is fixed by heating, application of pressure, thermal-pressurizing, or solvent vapor to thereby produce a copy. The toner remaining on the photosensitive member is removed by cleaning using various methods, and the above process is then repeated.
Furthermore, a general method for forming a full color image will be described. The photosensitive member of a photosensitive drum is uniformly charged by a primary charger, and image exposure is effected by a laser beam modulated by a magenta image signal of the original to form an electrostatic latent image on the photosensitive drum; the development of the latent image is then performed by a magenta developing device containing magenta toner to form a magenta toner image. Next, the magenta toner image developed on the photosensitive drum is transferred to a conveyed transfer material by a direct or indirect means using a transfer charger.
The photosensitive drum on which the development of the electrostatic latent image has been produced is subjected to charge dissipation by means of a charger for charge dissipation, and to cleaning by a cleaning means. Then, it is charged by the primary charger again, and the formation of a cyan toner image and the transfer of the cyan toner image to the transfer material, to which the magenta toner image has been transferred, is effected. Furthermore, similar processes are conducted successively using yellow and black toners to transfer four-color toner images to the transfer material. Fixing of the images to the transfer material having the four-color toner images is effected by heat and pressure using a fixing roller to thereby form a full color image.
Recently, such an apparatus is used not only as a copying machine for office use for simply copying an original but also as a printer for printing the output of a computer or as a personal copying machine.
In addition to laser printers, such apparatuses are rapidly being developed as printers for facsimile apparatuses using ordinary paper in which a basic engine is used.
As a result, a reduction in size and weight, an increase in speed, and an improvement in image quality and reliability are strongly demanded, and the components of such apparatuses have become simplified. Improved performance is required of the toner since no superior apparatus is possible without achieving an improvement in the performance of the toner. Furthermore, recently, various types of copying are necessary, and the demand for color copying is rapidly increasing. To copy the original color image more faithfully, a further improvement in image quality and resolution is required. From these viewpoints, it is necessary for the toner used in the color image forming method to exhibit satisfactory melting and mixing properties when heat is applied thereto, and it is desirable to use a toner having a low softening point, a low melting viscosity, and a high rapid melt property.
By using such a rapid melt toner, it is possible to widen the color reproduction range of copies and to obtain a color copy true to the original image.
However, generally speaking, such a color toner having a high rapid melt property has a great affinity for the fixing roller and tends to be offset on the fixing roller at the time of fixing.
In particular, in the case of the fuser of a color image forming apparatus, a plurality of toner layers of magenta, cyan, yellow, and black is formed on the transfer material, so that offsetting is liable to occur due to the increase in the number of toner layers.
Conventionally, for the purpose of preventing the toner from adhering to the surface of the fixing roller, the roller surface is formed, for example, of a material superior in release property with respect to toner, such as silicone rubber or fluoro rubber, and furthermore, to prevent offsetting and wear of the roller surface, the roller surface is coated with a thin film of a liquid having high release properties such as silicone oil or fluorine oil. In this case, the releasing agent and the roller surface are materials having great affinities. However, although this method is very effective in preventing offsetting of the toner, it is necessary to provide a device for supplying an offset preventing liquid, so that the fixing device is complicated. Furthermore, the application of this oil leads to mutual separation of the layers constituting the fixing roller, with the result that the service life of the fixing roller is shortened. The transfer material to which a toner image is transferred by using a fuser consists, generally, of various types of paper, coating paper, plastic film or the like. Above all, attention has recently focused on the need for a transparency film (OHP film) for use in an overhead projector for presentation purposes. Unlike paper, OHP film sheets are rather poor in oil absorbing capacity, so that the copy OHP sheets available at present are inevitably sticky when oil is applied thereto, which leads to rather poor quality of the obtained image. Furthermore, it is quite possible that the silicone oil, etc. will evaporate due to heat, thereby contaminating the interior of the apparatus, causing a problem of processing the recovered oil. In view of this, a method has been proposed according to which, instead of using a silicone oil supplying device, a releasing agent such as low molecular weight polyethylene or low molecular weight polypropylene is added to the toner so that an offset preventing liquid may be supplied from the toner during heating. However, if a great amount of this additive is added to achieve a satisfactory effect, filming of the photosensitive member occurs or the surface of the toner holding member such as the carrier and sleeve is contaminated, resulting in a deterioration in the image quality. In view of this, an amount of releasing agent, which is small enough not to deteriorate the image quality, is added to the toner, and some releasing oil is supplied or a device of the type which takes up the offset toner by using, for example, a web-like member is used, or a device which performs cleaning by using a cleaning pad is used.
It is well known in the art to add some wax to the toner as a releasing agent, as disclosed in, for example, Japanese Patent Publication No. 52-3304, Japanese Patent Publication No. 52-3305, and Japanese Patent Laid-Open No. 57-52574.
Furthermore, techniques in which wax or the like is added to the toner are disclosed in, for example, Japanese Patent Laid-Open No. 3-50559, Japanese Patent Laid-Open No. 2-79860, Japanese Patent Laid-Open No. 1-109359, Japanese Patent Laid-Open No. 62-14166, Japanese Patent Laid-Open No. 61-273554, Japanese Patent Laid-Open No. 61-94062, Japanese Patent Laid-Open No. 61-138259, Japanese Patent Laid-Open No. 60-252361, Japanese Patent Laid-Open No. 60-252360, and Japanese Patent Laid-Open No. 60-217366.
By using a toner containing wax, it is possible to prevent offset without having to apply a releasing agent to the surface of the roller as in the conventional art. Thus, the material of the surface of the fixing roller is not restricted to one having great affinity for the releasing agent. That is, in the case of the conventional rapid melt toner, the roller surface conventionally consists of silicone rubber or fluoro rubber. These involve problems with respect to wear resistance, strength, etc. When the number of copies to be made is high, flaws are generated on the surface of the roller, resulting in defective images. By using a toner containing wax, it is possible to use a fluoro resin, which is superior in wear resistance and releasing property, as the material of the roller surface layer.
However, using a fixing roller coated with such a fluoro resin produces the following problem.
The surface of such a roller coated with fluoro resin is harder than the surface of a conventional roller whose surface layer is an elastic layer, so that deformation of the surface of the fixing roller occurs to a smaller degree than in the case of the elastic layer roller, and the discharge direction of the recording material is higher in the case of a fluoro-resin coated roller than in the case of the conventional roller. When the discharge direction is higher, there is unevenness in gloss, or the recording material may be wound around the fixing roller, depending on the kind of recording material. In particular, when performing fixing on OHP film, it is necessary to conduct fixing at lower speeds in order to improve transmissivity, and the film may become easily wound around the fixing roller. In view of this, an elastic layer is provided beneath the surface fluoro resin layer to increase the thickness to thereby soften the fixing roller and increase the amount of deformation, whereby paper discharge is effected in a stable manner below the horizontal direction.
However, when a fixing roller constructed as described above is used, the surface resin layer cannot follow the elastic layer beneath it due to the difference in coefficient of linear expansion between the elastic layer and the surface resin layer, so that when the number of copies to be made increases and the surface resin deteriorates, cracks are generated in the surface resin, resulting in defective images. This is likely to occur when the thickness of the elastic layer is relatively large.